Thermal switch-heat pipe

ABSTRACT

A radial heat pipe is fitted with a wick and working fluid for the temperature of interest and operated in such a manner as to act as a thermal switch. The heat pipe surrounds nuclear fuel elements and during normal temperature operation transports negligible energy but upon a surge in fuel element temperature large amounts of heat can be transported to limit the fuel element temperature.

Elite States Gregorie et a1.

14 1 Dec. 17,1974

[ THERMAL SWITCH-HEAT PIPE [75] Inventors: Keith E. Gregorie; Henry C.

Pfefferlen, both of San Jose, Calif.

[73] Assignee: The United States of America as represented by the United States Atomic Energy Commission, Washington, DC.

[22] Filed: Sept. 7, 1972 21 Appl. No.2 287,211

[52] U.S. C1 165/32, 165/105, 176/38, 176/78 [51] Int. Cl. F28d 15/00 [58] Field of Search 165/105, 32

[56] References Cited UNITED STATES PATENTS 1,987,] 19 1/1935 Long 165/105 2,313,087 3/1943 Parr et a1 165/105 X 2,350,348 6/1944 Gaugler 165/105 X 3,229,759 H1966 Grover 1 165/105 3,405,299 10/1968 Hall ct al. 165/105 X FUELi PIN 3,490,718 1/1970 Vary 165/105 x 3,613,774 10/1971 Bliss, Jr 165/105 x 3,613,778 10/1971 Feldman, 11.... 165/105 3,638,023 1/1972 Cottam et al 165/105 x FOREIGN PATENTS OR APPLICATIONS 1,937,782 2/1971 Germany 165/105 Primary E.raminerAlbert W. Davis, Jr. Attorney, Agent, or Firm-John A. Horan; Frederick A. Robertson; L. E. Carnahan 5 7 ABSTRACT A radial heat pipe is fitted with a wick and working fluid for the temperature of interest and operated in such a manner as to act as a thermal switch. The heat pipe surrounds nuclear fuel elements and during normal temperature operation transports negligible energy but upon a surge in fuel element temperature large amounts of heat can be transported to limit the fuel element temperature.

4 Claims, 5 Drawing Figures EVAPORATOR 14' CONDENSER LITHIUM RESERVOIR PATENTEQ IE8 I 7 I874 SHEET 1 OF 5 III% e-FUEL PINS .--FLOWMETER RADIAL HEAT TRANSFER IC0I/cm. $ecI THROUGH LINER PATENTED 359 I 7 I974 '3. 8549524 SHEET IIIUF 5 I I I I000 I l LINER MoDIFIED I As LITHIUM I HEAT PIPE I I 100 uNMoDIFIED LINER wITH HELIUM I y FULL PowER H RADIATION 15 FULL POWER l I 534, FULL PowER 1 I I I! LITHIUM /I HEAT PIPE I I HELIuM- I uNMoDIFIED LINER WITH NEQN I /I CLAD MELTs 01 l I I I 1 I0 I00 4 RADIAL AI IFI THROUGH LINER (T minus T 1 THERMAL SWITCH-HEAT PIPE BACKGROUND OF THE INVENTION The invention described herein was made in the course of, or under, Contract No. AT(043 )-189, Project Agreement No. 10, with the United States Atomic Energy Commission.

This invention relates to safety systems for nuclear reactors, particularly to means for limiting the temperature of nuclear fuel due to loss of coolant, and more particularly to a type of thermal switch which transports negligible heat during normal operation but large amounts of heat during a temperature excursion.

A major safety-related area of concern in nuclear reactors, and more particularly the liquid metal fast breeder reactors, is the potential for rapid reduction or loss-of-coolant flow to localized portions of the core. The overall coolant system is similar to that of US. Pat. No. 3,525,669. Thus, one of the major hazards to be considered in a long term nuclear fuel irradiation in a forced circulation sodium-cooled system is the consequence of loss of flow. A power outage, pump failure, or flow blockage could each cause loss of sodium flow with possible overheating of the fuel elements unless there is some means of dissipating the heat in the vicinity of the fuel elements.

While reactor control systems have been developed in the prior art such that in the event of loss-of-flow the reactor is programmed to scram within a few miliseconds, decay heat and energy from delayed neutron induced fission will combine to generate about 12-15 percent of full power for a few seconds after reactor scram. Natural circulation of the sodium coolant cannot be counted on to dissipate this large an amount of heat. Conduction and radiation through existing coolant direction flow liners about the fuel elements are also inadequate. An even more serious situation would develop if the loss of coolant flow persisted more than a few minutes. Thus, there is a need in the prior art for an effective means of dissipating heat under temperature excursion conditions.

SUMMARY OF THE INVENTION The present invention provides a means capable of causing a large change in the thermal conductivity of a double walled structure by a relatively small change in the temperature of the hotter side of the structure. This is accomplished by a novel double wall structure which acts as an annular heat pipe. The working fluid of the heat pipe is such that in the temperature range of interest radial heat transfer through the double wall will vary from negligible to large. The heat pipe, for example, being operated exclusively in the startup regime of a nuclear reactor. The heat pipe is fitted with a wick and working fluid such that it acts as a thermal switch with negligible heat transported during normal operaity of a double walled structure by a relatively small change in the temperature of the hotter side of the double wall structure.

Another object is to provide a radial heat pipe containing a suitable wick and working fluid to transport negligible heat during normal operating temperatures but large amounts of heat during a temperature excur- SlOIl.

Other objects of the invention will become readily apparent from the following description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 schematically illustrates a forced sodium circulation nuclear fuel capsule utilizing the inventive heat pipe;

FIG. 2 partially in cross-sectionpartially illustrates the inventive heat pipe positioned about a nuclear fuel element;

FIG. 3 graphically illustrates the evaporation rate of lithium relative to increasing temperature;

FIG. 4 graphically illustrates the heat transfer capabilities of various liner designs as a function of temperature; and

FIG. 5 graphically illustrates the performance of a sodium heat pipe thermal switch made in accordance with the invention for various condenser temperatures.

DESCRIPTION OF THE INVENTION The invention is directed to a means capable of causing a large change in the thermal conductivity of a double walled structure by a relatively small change in the temperature of the hotter side of the structure, and is accomplished by a double wall structure which acts as an annular heat pipe. The working fluid of the heat pipe is selected so that in the temperature range of interest radial heat transfer through the double wall structure will vary from negligible to large. p

The invention is described and illustrated in an application for the transfer of large amounts of heat from fuel elements in a nuclear reactor during a temperature excursion while negligible heat is transferred during normal operation temperatures. However, the invention is not limited to this specific application and may readily be utilized wherever a similar heat transfer need arises. More specifically, the invention generally comprises a radial heat pipe which is fitted with a suitable wick and working fluid for the temperature of interest and operated in such a manner as to act as a thermal switch.

As pointed out above, one of the major hazards to be considered in long term nuclear fuel irradiation in a forced circulation liquid metal (e.g., sodium) loop is the consequence of loss of flow of coolant. A power outage, pump failure, or flow blockage could each cause loss of sodium flow with possible overheating of the fuel elements unless there is some means of dissipating the heat in the vicinity of the fuel element effected. V

FIG. 1 schematically illustrates a forced sodium circulation capsule utilized, for example, in irradiation tests of nuclear fuel pins or elements and comprises a primary containment 10, a secondary containment 11 positioned in spaced relationship within primary containment 10 so as to define a binary gas annulus l2 therebetween. A plurality of fuel pins 13 (three in this illustration) are located within a casing 14 which is centrally positioned in spaced relationship within secondary containment 11, to define a space 15 therebetween, and abuts at the upper end thereof, through appropriate sealing means, with a pump assembly generally indicated at 16, space 15 serving as a downward flow channel for the sodium coolant discharged from pump assembly 16 as indicated by the arrows, with the upward coolant flow being within casing 14 around fuel pins 13 as indicated by the upward directed arrows thereby producing a forced sodium circulation loop. A flow deflector 16' is mounted in pump assembly 16 to direct coolant discharge in a downward direction. A liner 17 is positioned in spaced relation about casing 14 defining a chamber or annulus 18 therebetween. The lower end of casing 14 is provided with a reduced diameter portion 19 so as to accommodate the positioning of a flowmete'r 20 between the lower end of liner 17 and reduced diameter portion 19 of casing 14.

In prior arrangements, the annulus l8 defined between casing 14 and liner 17 (hereinafter referred to as an unmodified liner) was filled with an insulating gas, such as neon or helium, thereby effectively inhibiting radial heat transfer through the liner 17.

In the event of loss-of-flow the reactor is programmed to scram within a few miliseconds. However, decay heat and energy from delayed neutron induced fission will combine to generate about 12-15 percent of full power for a few seconds after reactor scram. Natural circulation of the sodium cannot be counted on to dissipate this large an amount of heat. Conduction and radiation through the unmodified liner (casing 14 and liner 17) are also inadequate. A computer calculation has shown that the upper endof the fuel pins 13 would experience a thermal transient of about 500F. A more serious situation could develop if the loss of flow condition persisted more than a few (about 5) minutes. Freeze-up of part of the sodium loop would occur in that time and natural circulation would be lost whereby overheating of the fuel pins would be about 700-1 ,000F.

Referring now to FIG. 2 a portion of an embodiment of the invention is illustrated, which basically involves the modification of the casing-liner assembly (elements 14 and 17) of FIG. 1 to incorporate thereinto an annular heat pipe, and like elements to those of FIG. 1 will be given similar reference numerals. As shown, casing 14 and liner 17' are interconnected by a member 21 thus defining a closed end annular chamber 22 about the fuel pins 13 (only one shown), the lower end of the annular chamber 22 constituting a reservoir 23 for lithium orother suitable material. A wick 24 is secured to the inner surface of casing 14 and may, for example, consist of a layer of powdered stainless steel bonded thereto or several layers of fine mesh screen. Thus, the structure 14, 17 and 21-24 defines an annular heat pipe wherein the casing 14 constitutes the evaporator thereof and liner 17' constitutes the condenser of the heat pipe, with lithium, for example, as the working fluid in reservoir 23 serving to replenish the wick 24. Heat transfer (Q) is'indicated by the arrows across chamber 22. The sodium coolant flow indicated by arrows 25 and 26 is circulated, for example, in the same manner as described above in FIG. 1. While not shown in FIG. 2, the flowmeter 20 or other desired instrumentation may be attached to the member 21 in a manner similar to FIG. 1.

Lithium as the working fluid in reservoir 23 has the following advantages:

1. Good long-term compatibility with austenitic stainless steel up to 500C allows fabrication of the heat pipe from the material.

2. High surface tension in the liquid phase (about 350 dynes/cm) enables the lithium to climb the full height of the wick (25 inches in this embodiment) via capillary action and maximizes wetting of the wick by the lithium.

3. High heat of vaporization (4680 cal/gm) increases the heat content of the lithium vapor effectively decreasing the response time of the heat pipe.

4. Low thermal neutron absorption (0' for Li is 0.033 barnes).

5. Availability 6. Suitable vapor pressure at desired operating temperatures (980I250F).

In the application of the inventive concept described and illustrated, it is required that energy transport through the heat pipe be negligible during normal operation at the temperature set forth above (980l,250F). This is necessary to achieve accurate sodium calorimetry measurements for capsule power determination. However, in the event of a surge in fuel pin or element temperature, it is required that the heat pipe be capable of rapidly increasing energy transport. The calculation of energy transport as a function of vapor pressure in a heat pipe during the startup regime of a nuclear reactor of capsule, as illustrated, requires a knowledge of the evaporation rate of the working fluid as a function of temperature. FIG. 3 graphically shows how the evaporation rate of lithium goes up with increasing temperature.

The heat transfer rate through the heat pipe (elements 14', 17 and 21-24) is obtained by multiplying the mass transfer rate by the heat of vaporization of lithium 4680 cal/gm). FIG. 4 summarizes the heat transfer capabilities of various heat pipe designs as a function of the temperature of the i.d., (evaporator) surface assuming an average temperature of 980F on the o.d., (condenser) surface. During normal operation the total radial heat transfer through the abovedescribed lithium heat pipe would be about 1 percent of the total heat generation of a fuel pin, thus not enough to effect the precision of sodium calorimetry. In the event of a temperature excursion in the upward flowing sodium (see FIG. 2) the radial heat transfer through the lithium heat pipe is seen, as illustrated in FIG. 4, to increase rapidly and at an i.d., temperature of l,230F (an increase of 200F over normal operation) becomes equal to 15 percent of the normal full power heat generation rate of the fuel pin. Contrast this with the performance of the prior utilized insulating gas (neon or helium) or unmodified liner of FIG. 1. The radial heat transfer is very low for low radial temperature drops thereacross in the case of the prior liner design. However, as shown in FIG. 4, radial heat transfer in the prior design increases much less rapidly with increasing radial temperature drop thereacross than the inventive heat pipe even when the effect of radiation is added.

FIG. 5 shows how the operation of a sodium, rather than lithium as the working medium in the inventive heat pipe thermal switch would be effected by different condenser temperatures. In general, response of the so dium heat pipe is slower" for higher condenser temperatures. In other words, as condenser temperature is increased, a greater evaporator temperature increase is required for a given increase in radial heat transfer capability. The curves of FIG. 5 are normalized to the same starting point for convenience of comparison. A lithium heat pipe would show the same general behavior as the sodium heat pipe since the shapes of their vapor pressure curves are essentially identical.

In general, the proper matching of working fluid properties with operating temperature and heat transfer requirement will permit the application of the inventive thermal switch principle over a very large range of temperature and heat transfer requirements.

It has thus been shown that the present invention provides a means for causing a large change in the thermal conductivity of a double walled structure by a relatively small change in the temperature of the hotter side of the structure. Such a means basically comprises a heat pipe which functions as a thermal switch and has particular application for requirements where the heat transfer through the double walled structure must vary from negligible to large, such an application being in nuclear fuel elements where negligible heat is to be transported during normal operating temperatures but large amounts of heat need be transported during a temperature excursion.

While a particular embodiment of the invention and materials utilized therein have been illustrated and described, modifications will become apparent to those skilled in the art, and it is intended to cover in the appended claims all such modifications as come within the spirit and scope of the invention.

What we claim is:

1. An apparatus defining a thermal switch positioned about a nuclear heat source capable of producing a large change in thermal conductivity of a structure by a relatively small change in the temperature of the hotter side of the structure such that negligible heat is transported during a predetermined operating temperature of the structure while a large amount of heat is transported during a temperature excursion comprising: vertically positioned annular evaporator means defining a first wall member, annular condenser means positioned externally of and in vertical spaced relationship with respect to said evaporator means and defining a second wall member, means defining a third member interconnecting one end of said wall member of said evaporator means and one end of said wall member of said condenser means defining an annular chamber therebetween and forming an annular reservoir for working fluid at the lower end portion thereof, wick means located within said chamber and said reservoir and secured only to said wall member of said evaporator means, said wick means being selected from the group consisting of a plurality of layers of fine mesh screen and a layer of powdered stainless steel bonded to said wall member of said evaporator means, and a quantity of working fluid retained in said reservoir and in contact with a portion of said wick means, said working fluid being selected from the group consisting of lithium and sodium and of the type capable of climbing by capillary action and wetting said wick means.

2, The apparatus defined in claim 1, wherein said heat source comprises at least one nuclear fuel pin, said evaporator means of said apparatus being positioned in spaced relationship with said fuel pin defining a coolant flow path therebetween.

3. The combination defined in claim 2, wherein said apparatus surrounds at least one nuclear fuel pin such that said evaporator means defines an annulus therebetween for coolant flow therethrough.

4. The apparatus defined in claim 1, wherein said heat source comprises a plurality of nuclear fuel pins wherein said first wall member surrounds said fuel pins defining a coolant flow path therebetween.

* l l l 

1. An apparatus defining a thermal switch positioned about a nuclear heat source capable of producing a large change in thermal conductivity of a structure by a relatively small change in the temperature of the hotter side of the structure such that negligible heat is transported during a predetermined operating temperature of the structure while a large amount of heat is transported during a temperature excursion comprising: vertically positioned annular evaporator means defining a first wall member, annular condenser means positioned externally of and in vertical spaced relationship with respect to said evaporator means and defining a second wall member, means defining a third member interconnecting one end of said wall member of said evaporator means and one end of said wall member of said condenser means defining an annular chamber therebetween and forming an annular reservoir for working fluid at the lower end portion thereof, wick means located within said chamber and said reservoir and secured only to said wall member of said evaporator means, said wick means being selected from the group consisting of a plurality of layers of fine mesh screen and a layer of powdered stainless steel bonded to said wall member of said evaporator means, and a quantity of working fluid retained in said reservoir and in contact with a portion of said wick means, said working fluid being selected from the group consisting of lithium and sodium and of the type capable of climbing by capillary action and wetting said wick means.
 2. The apparatus defined in claim 1, wherein said heat source comprises at least one nuclear fuel pin, said evaporator means of said apparatus being positioned in spaced relationship with said fuel pin defining a coolant flow path therebetween.
 3. The combination defined in claim 2, wherein said apparatus surrounds at least one nuclear fuel pin such that said evaporator means defines an annulus therebetween for coolant flow therethrough.
 4. The apparatus defined in claim 1, wherein said heat source comprises a plurality of nuclear fuel pins wherein said first wall member surrounds said fuel pins defining a coolant flow path therebetween. 